A spectral image refers to an image in which each pixel is holding spectral information of an object. An RGB image developed from a spectral image excels in color reproducibility compared with an RGB image acquired by an RGB camera that is commonly used. Furthermore, a spectral image holds more information than an RGB image that is normally used. Spectral images are used in various fields, such as in the remote sensing and color design of digital cameras.
Conventionally various methods are known to acquire a spectral image of an object. One practical spectral image acquisition method that was invented estimates a spectral image from a few multi-band images using a statistical method. “Multi-band image” refers to an image group acquired by imaging an object using a plurality of optical filters each of which has a different transmission wavelength band. Hereafter an image captured in each transmission wavelength band of a multi-band image is called a “band image”, and the process of capturing a multi-band image is called “multi-band imaging”.